It is advantageous to combine a fluid flow rate measuring device with a valve. Such a device provides a compact means for precisely controlling the flow of fluid to readily establish a desired volumetric flow rate. One example of such a device is provided by positioning a venturi immediately upstream of a butterfly valve. Immediately upstream in this example means that the venturi is located within one or two pipe diameters of the valve closing member. While this configuration provides a compact design, the closeness of the venturi to the internal valve components does not allow accurate pressure measurements to be made from which the volumetric flow rate may be determined. Turbulence in the flow through the valve caused by the valve closing member (a plate or disk in a butterfly valve) causes the pressure in the venturi to fluctuate randomly, thereby reducing the ability to make accurate pressure measurements, and hence adversely affects the flow rate determination. The position and degree of turbulence is further varied in the venturi as the valve closing member is moved, making it difficult to adjust the position of the valve closing member in response to real time pressure measurements to obtain a desired flow rate.
There is clearly a need for a combination valve and flow rate measurement device which provides a compact design which also provides accurate flow rate measurements in close proximity to the valve.